1. Field of the Invention
The present invention relates to an imaging means for reading a document and an image reader using the same.
2. Description of the Related Art
Image scanners serving as image readers for reading image information on document surfaces have been disclosed in, for example, Japanese Patent Laid-Open No. 3-113961.
Such image scanners read image information by exposure-scanning the document surface, which involves moving only a reflecting mirror while an imaging lens and a line sensor are fixed.
In order to simplify the device structure, a carriage-integrated scanning system has been employed which scans a document surface with an imaging lens, line sensor and so on, which are integrated together.
FIG. 3 shows an image reader of a conventional carriage-integrated scanning system. Light flux, emitted from a light source 501, illuminates a document 503 placed on a document glass plate 502. The light flux reflecting off the document 503 passes through first, second, and third mirrors 504, 505, and 506 in order, such that the optical path is bent in a carriage 511. The light flux then passes through an imaging lens 509 to form an image on a line sensor 510. A drive motor (not shown) moves the carriage 511 in the direction of arrow A (in the subscanning direction) to thereby read the image information on the document 503. The imaging lens 509 is fixed to a lens barrel 512. The first, second, and third mirrors 504, 505, and 506 and the lens barrel 512 are fixed to the carriage 511.
It is important for image readers of digital copying machines to have high resolution and high speed. Increasing the optical path length can improve resolution of imaging lenses, but light intensity decreases because of a decreased angle of view. Further, making the imaging lens lighter, however, decreases resolution because of an increased angle of view. As such, it was therefore difficult to adopt the carriage-integrated scanning system.
Accordingly, an image reader of a carriage-integrated scanning system is disclosed in Japanese Patent Laid-Open No. 2002-335375 which adopts a decentered optical system (off-axial optical system) that forms an image by the reflection of light flux, as disclosed in Japanese Patent Laid-Open No. 8-292371. An off-axial optical system is defined as an optical system including a curved surface (off-axial curved surface) in which when the axis along a light beam passing through the center of an image and the center of the pupil is the reference axis, the plane normal at the intersection point of the constituent surface with the reference axis is not on the reference axis. The reference also discloses a structure in which air is used as the medium between the reflecting surfaces of the off-axial optical system to prevent the occurrence of chromatic aberration, particularly when color documents are read.
The above-mentioned references, however, do not disclose fixing means and adjusting means for the reflecting mirrors, the imaging mirrors having the off-axial reflecting surfaces, and the line sensors in the off-axial optical system adopted to the image readers of the carriage-integrated scanning system.
Image readers of digital copying machines can generally read documents having sizes from about A4 to A3. Thus, the off-axial reflecting surfaces need to be on the order of 30 mm each side. The off-axial reflecting surfaces are also required to have extremely accurate free-form surfaces. The working of the reflecting surfaces therefore requires several times of reflecting-surface measurements and correction processings. However, in the above references, the plurality of reflecting surfaces constructing the off-axial optical system is formed on one integrated optical element. Accordingly, in working the reflecting surfaces, when one reflecting surface is corrected, the free-form surfaces of the other reflecting surfaces are deflected, making it extremely difficult to finish all the surfaces as large as about 30 mm and with required accuracy. Furthermore, a hollow integrated block that uses air for the medium between the off-axial reflecting surfaces in order to reduce the chromatic aberration in reading color documents has a shape that is difficult to process. In other words, in the case of working the off-axial optical element by cutting, it is difficult for a cutting tool to reach the surface to be cut, and for die molding, it is difficult to separate the die from a molded optical element.